The ability of the yeast Saccharomyces cerevisiae to undergo meiosis and sporulation (gametogenesis) depends on the alleles present at the mating type locus, MAT. Our studies are directed at two aspects of this problem: an understanding of the nature of the MAT alleles and the identification and characterization of specific gene products whose synthesis is controlled both by sporulation conditions and the MAT locus. The mating type alleles a and alpha can replace each other by transposition from other, unexpressed loci where copies of a or alpha information are stored. We have isolated a number of mutations that block normal homothallic conversions and are examining them to determine how such switching occurs. By two-dimensional gel electrophoresis we have followed the relative rates of synthesis of both ribosomal proteins and generally abundant proteins, to show that only a very small number of proteins are controlled by the MAT locus, although a significant number respond to changes in the environment that promote sporulation. We are currently investigating which proteins are affected by a set of tempertaure-sensitive sporulation (SPO) mutants.